I will be as detailed on this as possible

I will be as detailed on this as possible. I have been having this problem over a period of three years. I have a 99 F-150 model XL 4WD, with a 4.2 Litre V6 in it. Every summer, when the outside temperature is above 90 degrees, my truck will do one of two things. First: It will not start. It will crank over but will have no spark. While it is doing this, the anti theft light will flash rapidly at me. Second: If the truck does start I can drive it fine, for maybe two blocks, then the truck will die, as if I have shut off the key. There is no sputter, and I till have all power, such as lights, radio, dash etc. When it does this, the anti theft light will flash rapidly. When the weather is below 90 degrees, my truck runs beautifully. I have taken it to two Ford dealerships, and both were paid to diagnose the problem. Both came up with the wrong diagnosis, and charged me to repair what they thought the problem was, only to have the problem continue. The problem is that the truck is not throwing any codes when this happens. I appreciate any help you can give me in this.